Control limits are one of the main elements of control charts. Generally speaking, a control chart is the graphical display of a statistic regarding the quality characteristic of interest, computed from a sample randomly drawn from a process at different time instances. As natural variability is always present in a process, we expect some variability on the control chart. Excessive variability, owing to special cause events, is referred to as being due to an assignable cause. Otherwise, when only chance causes – also called common causes of variation – are operating, the process is said to be in statistical control. In order to make a decision about the status of the process, control limits are typically positioned so that under the hypothesis of no deviation in the process, a type I probability error corresponds to an economically acceptable average run length. In this article, we discuss in a general manner how to compute control limits, and give some remarks on how limits should be computed so as to reduce false out of control signals.
Lingua originaleEnglish
Titolo della pubblicazione ospiteWiley StatsRef: Statistics Reference Online
Numero di pagine7
Stato di pubblicazionePubblicato - 2016


  • average run length
  • control charts
  • false alarm rate
  • process control
  • upper and lower control limits

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